Cylinder head

The air compressor cylinder head seals the compression chamber and controls the intake and compression strokes of the pneumatic braking system via intake and delivery valves. Together with the cylinder head bolts, this component determines the seal between the compressor cylinder, cylinder head gasket and coolant or compressed-air channels. This does not refer to the cylinder head of the combustion engine. For MAN and Mercedes trucks and buses, the correct compressor cylinder head must always be identified using the chassis number, compressor type and part number. Visual comparison or selection based solely on the model name is never sufficient.

Compressor cylinder heads are used on single-cylinder and twin-cylinder compressors, in air-cooled or liquid-cooled versions. Particularly on heavy-duty applications with MAN D20, D26 and D38 engines and Mercedes OM 471, OM 473, OM 470 or OM 501 engines, the head design varies according to compressor manufacturer, output class and installation position. Even on MAN TGL, TGM, TGS, TGX and TG3, Mercedes Atego, Axor, Actros and Arocs, and buses such as MAN Lion’s City, Mercedes Citaro and Neoplan coaches, compressed-air connections, coolant passages and mounting patterns are not interchangeable.

The cylinder head is generally made of cast iron or aluminium alloy and incorporates machined sealing surfaces, valve seats, cooling channels and compressed-air passages. Cylinder head bolts are torque-to-yield bolts or high-strength bolts with a specified tensile strength, thread pitch and clamping length. Their preload must withstand internal compression pressure, thermal expansion and vibrations without locally overloading the cylinder head gasket. An incorrect bolt length, damaged thread or incorrect washer will result in insufficient clamping force, even if the tightening torque appears correct.

A defective compressor cylinder head or cylinder head gasket often results in slow pressure build-up, a compressor that continues running for an extended period, audible compressed-air leakage, or traces of oil and coolant around the head joint. In liquid-cooled compressors, an internal crack can allow coolant to enter the compressed-air circuit. This puts strain on the air dryer, causes corrosion in air reservoirs and can contaminate control valves, EBS (Electronic Braking System) modulators and spring brake chambers.

First check the pressure build-up time according to the vehicle specification and compare the system pressure before and after the pressure regulator. Then connect a calibrated pressure gauge to the compressor delivery outlet. Insufficient delivery pressure may indicate leaking delivery valves, a burnt valve seat, a cracked head or a defective cylinder head gasket. Use soapy water or an ultrasonic leak detector to locate external leaks at the head joint. With the delivery line removed, check for excessive engine oil: in addition to worn piston rings, this may also indicate thermal damage to the compressor cylinder head.

Fully depressurise the air reservoirs, disconnect the battery and drain the cooling circuit on water-cooled versions. Measure the flatness of the sealing surface using a straightedge and feeler gauge; local distortion must not exceed the manufacturer's permitted tolerance. Also inspect delivery valves for carbon deposits, blue discoloration and broken spring reeds. Do not reuse a head with worn valve seats or corrosion pitting in the gasket surface.

Replace the cylinder head if cracks, distortion, defective valve seats or irreparable damage to cooling and compressed-air channels are found. Replace cylinder head bolts in case of corrosion, an elongated shank, damaged threads or where the manufacturer specifies torque-to-yield bolts. Thoroughly clean the bolt holes; liquid or debris at the bottom of a blind hole can cause hydraulic lock and cracking during tightening.

Always fit a new, suitable cylinder head gasket and follow the specified tightening sequence from the inside out. Use only the specified tightening torque and, where required, the additional angle tightening using a torque wrench and angle gauge. After installation, check the pressure build-up, the cut-out pressure of the compressor control system and any coolant leakage. Correct selection and technical inspection reduce unplanned downtime; Braem can assist with chassis-number-based parts identification.